Subarea II - Matter and Atomic Structure
Subarea II covers classification of matter, atomic models, quantum numbers, electron configuration, periodic trends, and nuclear change. Questions often ask you to connect a model or configuration to observable properties.
Matter and atomic structure
Pure substances are elements or compounds; mixtures are homogeneous or heterogeneous. Atoms are defined by atomic number Z (protons); isotopes differ in neutrons and mass number A = Z + N. Average atomic mass is a weighted average of isotope masses by natural abundance.
Quantum model
Electrons occupy orbitals named by n, l, m_l, and m_s. n = 1, 2, 3...; l = 0 to n−1 (s, p, d, f); each orbital holds two electrons with opposite spin. Fill order follows the Aufbau principle, Pauli exclusion principle, and Hund’s rule. Valence electrons drive periodic trends: atomic radius decreases left-to-right and increases down a group; ionization energy and electronegativity increase left-to-right and decrease down.
Nuclear chemistry
Alpha decay loses 4 mass units and 2 protons; beta-minus decay converts a neutron to a proton and emits an electron; gamma emission changes energy, not Z or A. Half-life problems use amount remaining = initial × (1/2)^(t/t½).
Worked example
Write the electron configuration of Fe²⁺ (Z = 26). Neutral Fe: [Ar]4s²3d⁶. For transition-metal cations, remove electrons from the highest-n shell first, so remove the two 4s electrons: Fe²⁺ = [Ar]3d⁶. Reasoning: 4s fills before 3d but is higher in n and is lost first; do not remove 3d electrons first.
Traps
Do not use mass number as average atomic mass. Do not assume 4s is always lower in energy for ions. Do not forget that beta decay increases Z by 1 while A stays the same. Do not confuse a group trend with a period trend.